We have the AARP Plan N. When we signed up, we were living in California, and the monthly cost difference was something like $95 for N and $150 for G. We were very healthy and might have 4 doctor visits a year, some of which had no copay as they were for "preventive" things. So we went with N.
We're now older and in Indiana, so I suspect the price difference is not so much. This plan is community-rated (preferable to age-rated) but there is a "discount" you get when you sign up at 65 that fades by a few percent every second year. Now at 72/74 we pay $282 per month total.
But the copay is not $20, it's "up to $20" and a lot of office visits are priced under $100, so the copay might be $14-18. Also, we have never seen an "excess charge" which is a 15% markup that some doctors are permitted to make, so that doesn't seem to be an issue.
So do the math. The other thing is that you are always permitted to change to a less-beneficial plan (e.g., G to N) but might need to undergo underwriting to go the other way, except in the states that permit it on request.
We have the cheapest (under $5 each) Aetna SilverScript Part D plan; 5 of our 6 meds are $5 for 3 months each. The other is a Tier 3 that is cheaper via GoodRx ($45), Amazon Pharmacy ($38), and now costplusdrugs ($18).